A Laboratory Test
for Williams Syndrome:
the FISH Test for the Elastin Gene
NOTE: In response to the many questions regarding the FISH test for the elastin gene, we have prepared the following general material. It should provide sufficient information to allow you and your family physician to decide whether or not you wish your child to take the test as a diagnostic for Williams syndrome.
The absence of an elastin gene from a chromosome is detected by the FISH technique. FISH is an acronym for the technical expression "fluorescence in situ hybridization." A blood sample is taken from your child and then treated with two specific colored markers that give off a "fluorescent" light when exposed to ultra-violet light. One of the markers attaches to each of the two copies of chromosome number seven in a cell. When both copies of chromosome possess the elastin gene, an additional fluorescence of another color is seen attached at another location to each of the two chromosome 7s. But, as is the case in over 95% of the Williams syndrome individuals tested, only one copy of chromosome seven, not two, will show the fluorescent spot for the elastin gene. If the FISH test shows the elastin gene to be missing from one copy of chromosome seven, then the individual tested can be said to have Williams syndrome.
In 95% or better of the cases studied, individuals having Williams syndrome are missing the elastin gene from one of their two copies of chromosome number seven. In Williams syndrome, there are at least 15 genes missing from one copy of chromosome seven, but the first one discovered, the elastin gene, is the one that is used in the current FISH diagnostic test.
If you have any doubts that your child may have Williams syndrome, the FISH test will give you a clear cut answer in most cases.
It may be possible, however, that an individual has most of the characteristics of Williams syndrome yet does not show that an elastin gene is missing. In such cases, only an experienced medical geneticist will be able to assess whether or not the questioned individual may have Williams syndrome.
You should also realize that it is extremely unlikely that you, your spouse, and your other children also have Williams syndrome. It is also unlikely that your other children have any greater chance of having children with Williams syndrome than anyone from the general population.
On the other hand, if your Williams child plans to become a parent, you should know that there is a 50/50 chance that his or her child will have Williams syndrome. If such a situation should arise, consult the obstetrician about using the FISH test for elastin for prenatal testing of the embryonic cells.
The test can be ordered by most genetics clinics or major hospitals. Most of the larger clinical laboratories will know how to order the test and will send the blood samples there for analysis. Costs may be covered by insurance or Medicaid or Medicare, but individual program and policy rules apply. Some will cover costs only with an order from the primary care physician. Some will not cover genetic tests at all, whereas others may cover only a portion of the cost.
